1. Field of the Invention
The invention relates to a high density optical disk on which recording and reproduction of information are conducted with respect to a land and a groove of a track, and also to an information recording/reproduction apparatus which conducts recording and reproduction of information on the optical disk.
2. Description of the Prior Art
An optical disk is a high density memory which has a large capacity, on which non-contact recording and reproduction can be conducted, and which can be replaced with another one. Capacities of typical optical disks are as follows: When an optical head of a lens NA of 0.5 is used, a 130-mm disk has a capacity of 300 to 500 MB per face, and a 90-mm disk has a capacity of about 128 to 250 MB per face. For a multimedia purpose, a high density recording/reproduction technique which uses a short-wavelength laser of 680 nm to obtain a capacity that is about 2 to 4 times the above-mentioned capacities has been studied.
FIGS. 10(a), 10(b) and 10 (c) show plan views (upper ones) and section views (lower ones) of continuous servo track formats in the prior art.
FIG. 10(a) shows a continuous servo track which is employed in a 130-mm or 90-mm optical disk of the prior art and which is of a land record track format. In the land record track format, a track consists of a groove 2 which is formed on a transparent substrate 1 and which has a depth of .lambda./(8.multidot.n) (.lambda. is the wavelength of a laser, and n is the refractive index of the substrate 1. The same applies to the followings.), and pits 4 constituting a sector identification (ID) signal, and record marks 5 are recorded onto a land 3 sandwiched by the tracks. The pits 4 for the ID signal are convex-concave pits having a phase depth of .lambda./(4.multidot.n).
The track pitch is selected so as to be about .lambda./NA which is obtained from the laser wavelength (.lambda.) and a lens aperture (NA). In the prior art disk, since the lands must remain to exist between the grooves 2 and the ID signal pits 4, it is difficult to reduce the track pitch to 1.3 .mu.m or less in the view point of a land forming process.
FIG. 10(b) shows an example of a groove record track format in which tracks are formed by simple grooves 6 having a phase depth of .lambda./(8Nn) and lands 7, and pits 8 constituting an ID signal and record marks 9 wherein data signals are recorded into the grooves 6. Since such a groove record track has a track structure consisting of the simple grooves 6, a disk of a track pitch which is 1 .mu.m or less can easily be produced.
FIG. 10(c) shows an example of a land/groove record track format in which groove record tracks are formed by setting the width of grooves 11 having a depth of about .lambda./(8.multidot.n) to be one-half of the track pitch, and signals 12 are recorded also onto lands 10. In principle, this land/groove recording can achieve an area recording density which is twice that of the land recording of FIG. 10(a).
Generally, when a track pitch is to be reduced, there arise problems in a cross-talk of a signal recorded in an adjacent track, in a cross-erase in which also signals into two adjacent tracks are erased by conducting a data recording, and in the stability of a tracking servo.
The stability of a tracking servo will be discussed. In the land record track 10 for the land/groove recording of FIG. 10(c), the track pitch is one-half of .lambda./NA. In the case where .lambda. is 830 nm and NA is 0.5, even when the pitch of tracks for recording signals is 0.8 .mu.m, the tracking servo can be conducted with respect to the track pitch of 1.6 .mu.m consisting of each groove and land. Consequently, the tracking can be done stably by either of the 3-beam method and the push-pull method which are conventionally employed.
Even in the land/groove recording in which recording is conducted as described above, when the track pitch is further reduced in order to increase the recording density, there arises a problem in that a cross-talk between the track of the groove 11 and that of the land 10 occurs.
Specifically, in the case where an optical head in which the laser wavelength .lambda. is 830 nm and the lens aperture NA is 0.5 is used, a cross-talk of -30 to -35 dB occurs when the track pitch is 1.6 .mu.m, and that of -15 to -20 dB occurs when the track pitch is 0.8 .mu.m, thereby producing a problem in that an ID signal and a data signal cannot be reproduced normally.
Particularly, there arise the following problems: In a track search process, the confirmation of a target track is made difficult by reproduction errors such as that an ID signal is caused to be erroneously reproduced by an effect of a cross-talk on the ID signal, and that an ID signal of an adjacent track is erroneously reproduced. In a recording process conducted on an unrecorded sector, even when an ID signal leaks from an adjacent track, the reproduction is apparently conducted in a normal manner, thereby causing data to be recorded into a wrong sector.
In the groove/land recording conducted as described above, however, the track density is doubled, and hence there arises a problem in that, in a recording or reproducing process, a cross-talk, or a cross-erase produces errors which exceed a criterion, so that defective sectors are generated in an increased number.